Silica aerogels are inorganic solid materials with high porosity and low density, and they are excellent thermal insulators. Due to their outstanding ability as thermal insulators, silica aerogels have great potential as building materials. They also have many civil applications in energy-saving such as storage media, catalysis etc.
Silica aerogel was first investigated by S. S. Kistler in 1931, who suggested replacement of liquid by gas in the pores of silica gels, to retain the solid porous silica portion of the gel porous structure against the capillarity. The conventional syntheses of silica aerogel use a drying method involving displacement of the liquid of the gels by a supercritical fluid of carbon dioxide and requires both high pressures and high temperatures (Anderson et al., Hydrophobic silica aerogels prepared via rapid supercritical extraction. Journal of Sol-Gel Science and Technology, 2010, 53, 199-207.). Such processes thus require large amounts of energy.
In 1989, a freeze drying process for the production of silica aerogels was developed (Klvana et al., A New Method of Preparation of Aerogel-Like Materials Using a Freeze-Drying Process. Journal De Physique, 50, 1989, C429-C432; Egeberg and Engell, Freeze drying of silica gels prepared from siliciumethoxid. Journal De Physique, 50, 1989, C423-C428). However, the freeze drying methods require the use of low temperatures and for this reason they are also energy intensive processes.
Ambient pressure drying (APD) provides an alternative, less energy intensive, option for the synthesis of aerogels (Prakash et al., Silica Aerogel Films Prepared at Ambient-Pressure by Using Surface Derivatization to Induce Reversible Drying Shrinkage. Nature, 1995, 374, 439-443.). Conventional APD methods rely on displacing the original solvent used for gel preparation with various organic solvents which have lower surface tension, such as hexane, heptane, octane and nonane. In most conventional APD approaches, additional surface modification is also introduced to replace the —OH groups on the silica surfaces with more lipophilic groups, in order to provide an additional reduction in capillarity (Rao et al., Hydrophobic and physical properties of the ambient pressure dried silica aerogels with sodium silicate precursor using various surface modification agents. Applied Surface Science, 2007, 253, 6032-6040; Jung et al., The properties of silica aerogels hybridized with SiO2 nanoparticles by ambient pressure drying. Ceramics International, 2012, 38, S105-S108.). A common reagent used for the surface modification of silica gels is trimethylchlorosilane (TMCS), (CH3)3SiCl. Surface modification by TMCS used in conventional APD process always leads to the generation of an acid (in the case of TMCS, HCl) so further organic solvents are needed in order to remove generated acid. Although less energy intensive than supercritical CO2 processes, current APD processes are time consuming and costly due to the need for large amounts of organic solvents. The high cost associated with the use of organic solvents limits the use of aerogels on a large industrial scale, for example as building materials.
It is an aim of certain embodiments of this invention to provide a method of producing silica aerogels which is cheaper, quicker and/or more energy efficient than the prior art processes. It is an aim of certain embodiments of this invention to provide a method of producing silica aerogels which is more environmentally friendly than prior art processes.